Ion mobility based analyzers, such as ion mobility spectrometers and differential mobility spectrometers, analyze ions based on the ions' mobility characteristics while the ions are flowing through a gas or mixture of gases. An ion mobility spectrometer (IMS) typically uses a voltage gradient to propel ions along a drift region toward a detector. A time-of-flight (TOF) IMS separates and discriminates among different ion species by measuring the arrival time of the different ions species at a detector because ions species having different ion mobility characteristics travel through a drift gas at different rates.
A differential mobility spectrometer (DMS), also referred to as a Field Asymmetric Ion Mobility Spectrometer (FAIMS), also analyzes ions that are flowing through a gas or mixture of gases. However, unlike an IMS, a DMS subjects the ions to a time-varying (e.g., asymmetric) field as the ions flow through an analytical gap between filter electrodes that apply the asymmetric field. The asymmetric field typically includes a high field period followed by longer low field period. A compensation field is also typically generated one of the filter electrodes (by applying a DC compensation voltage to the electrode) that enables the DMS to pass through a selected ion species. Other species are typically deflected toward one of the filter electrodes and neutralized. Ion mobility based analyzers, such as an IMS or DMS, are capable of identifying samples and sample constituents by measuring an ion intensity spectrum and comparing that spectrum with a known spectrum or spectra.
DMS or DMS analyzers typically have a cylindrical or planar form factor. Cylindrical DMS analyzers, such as those described in U.S. Pat. No. 6,621,077, employ a terminus and trap region to enable the collection and concentration of ions before introduction of the ions from the DMS or DMS into an MS. One problem with this structure is that ions tend to be distributed or diffused by the cylindrical DMS filter which results in the need for a terminus and ion trap to concentrate the ions before introduction into an MS.
Mass analyzers or Mass Spectrometers (MS), unlike ion mobility based spectrometers, measure the mass-to-charge ratio (m/z) of ions by subjecting ions within a vacuum to an accelerating electric field. In a TOF MS, ions having different mass-to-charges ratios are subjected to the same electric field. Because different ion species have different mass-to-charge ratios, the different ion species undergoes different amounts of acceleration and, therefore, arrive at a detector at different times. Hence, a TOF MS is capable of detecting and measuring different ions based on their different mass-to-charge ratios. A MS can identify the components of a sample by determining their molecular weight or mass.
Chip-based or micromachined IMS, DMS, and MS systems are commercially available today. Such micromachined systems are desirable because they enable the use of compact and portable ion detection systems.
IMS, DMS, and MS analyzers often operate as stand-alone systems. However, certain types of combined analyzers such as a tandem IMS-MS, tandem DMS-MS, or tandem IMS-DMS-MS system may be employed. For example, Thermo Fisher Scientific, Inc., of Waltham, Mass., markets a cylindrical DMS (FAIMS) interface that can be interfaced with their TSQ Quantum® series mass spectrometers for laboratory research.
One problem with using a cylindrical DMS or DMS as a pre-filter to a MS is that researchers must attach the cylindrical DMS per-filter to the MS when pre-filtering is desired, but then disconnect and remove the cylindrical DMS pre-filter from the MS when analysis without DMS pre-filtering is desired. This is necessary because, as stated above, the cylindrical DMS structure tends to diffuse ions. Thus, if the cylindrical DMS filter is only de-activated without removal, the ions within a cylindrical DMS will no longer be concentrated or trapped prior to entry into the MS, resulting in degraded system sensitivity and performance. Thus, the cylindrical DMS must be disconnected from the MS to prevent the ion diffusion effects of a deactivated cylindrical DMS before ion introduction into the MS. The attachment and detachment requirement of the cylindrical DMS is undesirable for numerous reasons: 1) attachment and detachment may expose the user to sample contamination, 2) attachment and detachment is time-consuming, 3) attachment and detachment requires user training, 4) attachment and detachment may result in excessive wear and failure of the DMS-to-MS connection, 5) attachment and detachment may reduce the reliability of the system, and 6) the detachable DMS interface may be lost or damaged when separated from the MS. Accordingly, there is a need for a DMS or DMS pre-filter to an MS that can be deactivated instead of disconnected when MS analysis without DMS pre-filtering is desired.
Another problem associated DMS-MS analyzers is that the relatively high transport gas flow rate of a DMS can result in a relatively high flow rate into the attached MS. Because an MS must maintain a high vacuum, a relatively powerful and, therefore, large pump is required to maintain such a high vacuum at the relatively high ion flow rate. While the size of a DMS-MS system may not be a concern in a laboratory environment, DMS-MS system size and power requirements are critical for portable, field-deployable, or in-situ sample analysis applications and uses. Accordingly, there is a need to reduce the size of the vacuum pump used for the MS to realize a more compact, portable, and less power-consuming DMS-MS system.